Tokyo Electric Power Company says this is the worst leak so far in terms of volume, according to the BBC.

Officials declared it a level-one incident, the second lowest level, on the International Nuclear and Radiological Event Scale. It marks the first radiological release incident since the earthquake and tsunami that severely damaged the plant in March 2011.

In a press conference Tuesday, Tepco officials explained the water may have leaked from a tank after breaking through a concrete barrier. A spokesman said that much of it already seeped into the soil and could eventually reach the ocean, reports the New York Times.

One puddle emitted 100 millisieverts of radiation an hour, which is equal to a five-year dose of radiation, said a Tepco general manager.

Levels of radioactive cesium and strontium higher than the legal safety limit are present in the leaked water, and exposure is known to increase the risk of cancer.

Workers placed sandbags around the leak and were pumping out the tank's remaining water to transfer it to other containers.